Allylphenols synthesized from phenols and allyl chloride and diallylbisphenols synthesized from bisphenols and allyl chloride have conventionally been known. Further, various resin compositions employing these allylphenols as hardeners have already been known such as, for example, a resin composition comprising an allylphenol and a maleimide compound (JP-A-55-39242), a resin composition comprising an allylphenol, a maleimide compound, and an epoxy resin (JP-A-53-134099), and a resin composition comprising an allylphenol, a maleimide compound, and a hydrazide. (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, use of conventionally known allylphenols as hardeners for the above resins has been defective in that the compositions should be heated at high temperatures for long periods of time in order to complete crosslinking reactions and that crosslinked resins (cured products) produced are insufficient in heat resistance, flexibility, and impact resistance.
Known as the most common method for producing an allyl ether of a phenol is a process comprising dissolving a phenol in acetone, subsequently converting the phenol into a phenolate by use of an equimolar amount or a slight excess of potassium carbonate, and then reacting the phenolate with an equimolar amount of allyl bromide [Journal of American Chemical Society, Vol. 62, pp. 1863 (1940)]. The above reactions are completed by 5- to 10-hour stirring to give the intended allyl ether in a yield of 80 to 100%. Further, also known are a method that is the same as the above process except that allyl chloride or a combination of allyl chloride and sodium iodide is used in place of allyl bromide and a method that is the same as the above process except that sodium carbonate, sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide, or the like is used in place of potassium carbonate. As the reaction medium for the above process, various organic polar solvents may be used besides acetone. Examples of such solvents include ketones such as methyl ethyl ketone and methyl propyl ketone (2-pentanone) and aliphatic alcohols such as methanol, ethanol, and n-propanol. (See, for example Organic Reactions (1944), Vol. II, pp. 22-28).
The above-described process for allyletherification employing a polar solvent such as those mentioned above, however, has been defective from the standpoint of industrial application because of the following disadvantages. Since inorganic salts (for example, NaCl, KCl, NaBr, KBr, etc.) formed as byproducts in the reactions generally are only slightly soluble in the polar solvent, the process necessitates a step of filtering off the inorganic salts after completion of the reactions. Moreover, in order to prevent the solution containing the allyl ether produced by the reactions from being emulsified when the ether is washed with pure water, the polar solvent, which is miscible with water, should be removed beforehand by evaporation and an only slightly water-soluble extraction solvent such as ether, benzene, toluene, ethyl acetate, methyl isobutyl ketone should be newly added. Furthermore, since the polar solvent is not so good in the ability to dissolve therein the base added and the phenolate generated, the process is not suited for industrial production in which the allyl ether is produced at a high concentration on a large scale.
Known as a method for producing an allyl-substituted phenol compound is a process in which an allyl ether of a phenol is subjected to an allyl-group rearrangement reaction, which is called the Claisen rearrangement, to produce a compound having an allyl-substituted aromatic nucleus. In this process, an allyl ether of a phenol is heated to isomerize the allyl ether to an o-allylphenol (a p-allylphenol if both ortho positions have a substituent). It has also been known that this rearrangement reaction generally proceeds easily by heating the allyl ether at a high temperature around 200.degree. C. for several hours to several tens of hours in the presence or absence of a high boiling point solvent such as Carbitol, ethyl cellosolve, N,N-diethylaniline, N,N-dimethylaniline, tetralin, kerosene, and paraffin oil. (See, for example, Organic Reaction (1944), Vol. II, pp. 29-48).
However, because the rearrangement reaction is carried out at such a high temperature, allyl groups undergo heat polymerization and other side reactions to yield undesirable by-products, even though allyl group is far less apt to undergo polymerization than vinyl group, etc.
Although it has also been known that the rearrangement reaction is conducted under reduced pressure or in an atmosphere of carbon dioxide, nitrogen, etc., (see page 24 of the above-mentioned literature), the effect of suppressing side reactions has been insufficient.
In addition, although the use of a solvent of the above-mentioned kind, such as N,N-diethylaniline, tetralin, kerosene, and paraffin oil, serves to suppress polymerization reactions and improve selectivity and yield for obtaining the intended allylphenols (see page 24 of the above-mentioned literature), there are problems that such effects can generally be produced only when the solvent is used in a large quantity and that the solvent, especially diethylaniline, must be separated and removed after the rearrangement reaction by means of distillation or extraction with an aqueous solution of a mineral acid or by other means. Thus, the process employing a solvent is unsuited for industrial applications.
The Claisen rearrangement may be performed without such a high boiling point solvent, but this method has been defective in that because the reaction is carried out at a high concentration, intermolecular reactions are liable to proceed and, as a result, byproducts are formed by side reactions such as heat polymerization in large amounts as compared with the case employing a solvent.
On the other hand, as another method for the allylation of phenols, a process in which a phenol is allylated with an allyl halide in an aqueous medium is known (J.A.C.S., 85, 1141 (1963)). In this process, in which a phenol or alkylphenol is reacted with an equimolar amount or less of an allyl halide, an aromatic-nucleus allylation reaction (C-allylation) takes place along with allyl-etherification (O-allylation) and the allylated product obtained is in the form of a mixture thereof. The allyl-substituted phenol obtained by this process also has a problem that where it is used as a hardener for a maleimide resin, the cure of the resin takes much time and the resulting cured products show insufficient performance.